Multiple TWTs are used on spacecraft and generally an individual power supply is associated with each TWT. The power supply delivers the necessary bias voltages and currents to the TWT that is essentially a high power amplifier. An example of a typical TWT with the basic requirements includes a filament, cathode, anode, collectors and helix. The filament voltage is applied first to preheat the filament. Once a predetermined temperature has been reached, the cathode, anode and collector voltages are applied. The helix is tied to ground potential.
In prior art TWTs the cathode current, or beam current, is controlled by the voltage difference between the cathode and the anode. The cathode current determines the maximum output power of the TWT. In the above example, the collector voltages are simply bias voltages.
Prior art power supplies for TWTs operate in two modes: ON or OFF. When operating in the ON mode, the maximum beam current is supplied, which allows the TWT to be driven to its maximum power. However, if the TWT is operated at less than maximum power, the power efficiency is reduced because of the fixed bias loss in the maximum setting of the beam current. Until the introduction of the present invention there has been no variable control of the TWT's beam current. If the beam current can be reduced for lower power applications, then the fixed bias loss is reduced thereby increasing overall efficiency. In space applications, TWTs are a major consumer of solar cell power because when they are ON they are running at full power. It is particularly advantageous to provide variable control of the TWT's output beam current in order to optimize a spacecraft's power budget.
Additionally, TWTs are generally powered from individual power supplies in order to achieve independent control. A significant weight and cost savings could be achieved by reducing the number of power supplies used in the space application. Independent ON/OFF control of multiple TWTs that are powered from a single power supply is highly desirable.